Just having Jan,Feb etc and trying to sort wont work because it will do a character sort.
Suggest having numbers along with it.
create table month(mth int, mth_str char(3))
insert into month values(1,'Jan')
and then use order by mth.
Also, not sure what the larger scope of your issue is. There are a lot of date functions which you can use along with maybe a date field. Check sybooks.sybase.com and use the reference manual/functions.